Previous Topic :Structure Of A Cell
Environment Of The Cell
In a living organisms, there is constant movement of material within a cell, materials such as enzymes, food, oxygen, carbon dioxide move from one part to another between a cell and its external environment.
The exchange of materials between an organism and its external environment is necessary for the life of the organism.
For example spirogyra.
Spirogyra lives in water, it obtains mineral salts, water, carbon dioxide and oxygen from its environment which are used for its vital activities.
In multicellular organisms, there is exchange of materials between different parts of one cell, between cells and the tissue fluids that surround them, between cells and their neighbouring cells as well as between cells and their external environment.
Diffusion : is the movement of molecules from a region of higher concentration to a region of lower concentration.
The molecules that make up matter are continually in motion, the speed with which the molecules move varies.
If you open a bottle of petrol or ethanol for a few seconds in one corner of a room, the petrol or ethanol evaporates and the molecules from the bottle spread widely to the other parts of the room. This spreading of molecules happens much faster in gases and vapours than in liquids.
Note : Diffusion is so slow or non-existent in solids.top↑
The movement of the molecules continue to spread until the source of the molecules are withdrawn and evenly dispersed throughout the space available to them.
The individual molecules are invisible to human eye, but it is however possible to carry out experiments that demonstrate the diffusion of molecules from a region of higher concentration to a region of lower concentration.top↑
Demonstration Of Diffusion In Liquids
Materials needed for the experiment are : Measuring cylinder of minimum capacity of 500cm³, distilled water, concentrated copper (ii) tetraoxosulphate (iv) (copper sulphate) solution, and a pipette.
a] Locate a spot in the laboratory where you can have the measuring cylinder standing undisturbed for weeks.
b] Pour 200cm³ of distilled water into the measuring cylinder. Pipette 100 cm³ of the copper solution.
Carefully lower the tip of the pipette to the bottom of the measuring cylinder and slowly run in 100 cm³ of the copper sulphate solution. If this is done correctly, there should be a distinct line between the clear distilled water above and the blue copper sulphate solution below, slowly withdraw the empty pipette from the solution.top↑
c] Cover and leave the measuring cylinder to stand undisturbed for about two weeks. Observe daily and record your observations.
During the observation, pay much attention to the boundary between the water and the copper sulphate solution.
d] After few days, you should observe a mixing up or shading off colour at the face of the solution and a blue colour below with a clear and colorless water above. If the cylinder was left for long enough, the colour will become mixed all through the liquid.top↑
At the end of the experiment, the copper (ii) tetraoxosulphate (vi) (copper sulphate) solution will be found to have diffused evenly through the water and the water molecules will also be found to have moved into the copper (ii) tetraoxosulphate (vi) solution. Hence, this proves that diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Demonstration Of Diffusion In Gases
Materials needed for the experiment are : Test tube, filter or tissue paper, dilute hydrochloric acid (HCl), litmus solution, rubber band and bottle of ammonia solution.
a] Filled the test tube with water to about three quarter of the test tube. Add a few drops of litmus solution to the water and add one or two drops of dilute hydrochloric acid (HCl) to the mixture. Make sure that the mixture turn to red before proceeding, if not add more hydrochloric acid to the solution.
b] Add more water to the mixture till the test tube is completely full, then place a filter paper or a piece of tissue paper over the mouth of the test tube, secure the paper tightly over the test tube with a rubber band.
c] Now the mixture will have been slightly acidic, invert the test tube over an open bottle of ammonia solution (NH3) and hold the test tube in an upright position for a few minutes.top↑
Ammonia is poisonous, so keep your face away from the ammonia bottle and avoid breathing in the ammonia fumes.
Record how soon you observe a change in the contents mixture after placing the test tube over the bottle of ammonia solution.
If you carry out your experiment and observation accurately, you should see the contents of the test tube changing colour gradually from red to blue.
The change started at the mouth of the inverted test tube and progressed upwards all through the test tube.
Molecules of ammonia gas had diffused through the liquid mixture and turned the original acid mixture to alkaline. This is observed through the colour change of the solution from red to blue.top↑
Note : This movement of molecules from one region to another region within a space depends on the relative concentrations of the molecules in different parts of the space.
Diffusion is responsible for the transportation of substances from one region to another region within the cell.
Factors That Affects The Rate Of Diffusion
a] State of matter : Diffusion of gases is faster than diffusion of liquids because gas molecules are much less tightly bound together to each other than in liquids molecules where molecules are tightly bound together.
Therefore, the rate of diffusion through gases is faster than the rate of diffusion through liquids.
b] Molecular size : the size of the molecules that make up a liquid or gas affects the rate of its diffusion. Larger molecules diffuses more slowly than smaller molecules.
c] Temperature the higher the temperature, the higher the speed at which molecules move, and the higher the speed of molecules, the faster the rate of diffusion. hence, the higher the temperature, the faster the rate of diffusion.top↑
d] The nature of the medium : The rate of diffusion is faster in gaseous medium than in liquid medium, and faster in liquid medium than in solid medium.
e] Concentration : The relative concentrations of molecules of a substance affects the rate of diffusion of a substance molecules. The higher the concentration of the substance molecules, the faster the rate of diffusion, this factor was expressed as a law called Fisk's lawtop↑
Fisk's law : states that the higher the concentration gradient of a molecule the greater the rate of diffusion.
Imbibition And Diffusion
Imbibition : is the movement of water molecules into substances causing increase in volume (swelling) of a substance as a result of the interaction between the substance molecules and the water molecules.
For example, when a thin piece of dry gelatin is placed in water, it swells, that is, its volume increases due to the water that passes through the gelatin molecules, the water molecules pushes apart the gelatin molecules and diffuses into the space between the water molecules and the gelatin molecules. Hence, the gelatin swells up and become bigger than the original dry gelatin.
In a solid or a dense colloid, the molecules are very close together which leads to limited movement of their molecules.top↑
Imbibition is the explanation for wooden doors becoming difficult to shut or open during wet seasons, it is the Imbibition that leads to a tremendous building pressure.
It is also the Imbibition that makes the bean seed to swell up when left in water for some time and also accounts for rice-laden ships which split when water goes into the rice.
Advantages Of Diffusion
a] Diffusion plays a very important role in the activities of a cell. When molecules are unevenly distributed in a cell, it is the diffusion that equalise the distribution of molecules all within the cell.
b] Diffusion also enhance exchange of substances within the cell and its environment.
Processes In Which Diffusion Occurs In Animals
a] Diffusion enhances gaseous exchange in amoeba, gills of fish, skin of toad and alveoli of the lungs.b] Diffusion also exchange nutrients between a foetus and the mother through the placenta.
c] Diffusion also absorbs food from the small intestine through the villi into the blood stream.top↑
Process In Which Diffusion Occurs In Plants
a] Diffusion transports carbon dioxide from the atmosphere into the leaves through the stomata, carbon dioxide which are used for photosynthesis.
b] Diffusion also transports oxygen that are produced during photosynthesis from the leaves to the atmosphere.
Note this is one of the intermediation between plants and animals (including human being). They produce oxygen that animals breathe in and animals produce them with carbon dioxide which were breath out by animals during respiration which plants used for photosynthesis.
c] Diffusion, movement of oxygen for respiration from the atmosphere into the plant through the stomata or lenticles.top↑
d] Diffusion transports manufactured food from the leaves to the other part of the plant.
e] Diffusion also transport carbon dioxide produced during respiration from the plant to the atmosphere through the stomata or lenticles.
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